The predominant market for magnesium metal currently is in the alloying of aluminum. The strength and light weight of certain magnesium-containing aluminum alloys makes the alloys well suited for use in various aerospace, automotive, and electronic components. Magnesium metal also is commonly used as a desulfurization agent in processes for refining ferrous metals, as well as in the production of titanium and zirconium metal. In the well-known Kroll process for producing titanium metal, TiCl4 is reduced to titanium metal by reaction with an excess of liquid magnesium at high temperature according to the following equation:2Mg(l)+TiCl4(g)→2MgCl2(l)+Ti(s)The magnesium chloride product can be further refined back to magnesium. The porous metallic titanium sponge produced in the reduction process may be purified by leaching or heated vacuum distillation.
Since the 1950's, the industrial production of zirconium metal has principally relied on the use of magnesium as a reducing agent. In typical zirconium metal production methods, approximately one part of magnesium (by weight) is required as a reducing agent to yield one part of zirconium metal sponge from zirconium (IV) chloride (i.e., zirconium tetrachloride) according to a well-known adaptation of the Kroll reduction process. Given the significant amount of magnesium required in the Kroll process per unit zirconium metal produced, at least a portion of any impurities present in the magnesium will be incorporated into the zirconium product. Therefore, it is important to carefully control the quality of magnesium used in the Kroll process in order to produce a highly pure zirconium product.
Impurities that are of concern in zirconium production include, for example, iron, aluminum, and nitrogen, and all of these elements may be present as impurities in a magnesium reductant. Iron is a common material used in the construction of magnesium refining equipment, and although iron has a relatively low solubility in molten magnesium (approximately 0.12 weight percent at 800° C.), this impurity level still represents a significant potential contributor to iron impurities in zirconium metal produced by the Kroll process. Aluminum contamination in magnesium reductant may originate from aluminosilicates entrained in brines used as starting material in magnesium production. Nitrogen impurities can form in magnesium when liquid magnesium contacts ambient air and, despite cover gases used in the course of magnesium refining, significant opportunities exist for this mode of nitrogen contamination.
Zirconium production, unlike many other processes in which magnesium is used, requires meeting strict limits on the levels of impurities. Top-quality zirconium metal is highly pure and unalloyed with other elements, and achieving this level of purity demands judicious management of starting materials. As examples, top-quality zirconium includes less than 1000 ppm iron and less than 100 ppm aluminum. As new alloys are developed and as zirconium customers seek to improve their products over time, the impurities limits for zirconium are expected to become even more restrictive. Nitrogen is an especially deleterious impurity in zirconium because it forms nitrides with zirconium. Excessive nitrogen can lead to large zirconium nitride regions, which are insoluble during zirconium melting and may significantly reduce product quality. Zirconium nitride inclusions in a cast zirconium metal are relatively hard regions and can be the source of voids or cracks as the zirconium metal is worked.
Accordingly, it would be advantageous to provide a method for reducing impurities in magnesium used as a reductant in the production of zirconium metal by the Kroll process, thereby improving the purity of the zirconium metal product. More generally, it would be advantageous to provide an improved method for reducing impurities in magnesium provided for any end use.